Throughout the specification, the expressions "earth" and "earth formation" are used in a broad sense to denote any nonhomogeneous, dispersive medium having complex permittivity. Important examples of such media include subterranean formations such as soil, hardpan, and bedrock, and bodies of water (such as the ocean) above such subterranean formations.
Information regarding the electrical properties (e.g., resistivity, conductivity, relative permittivity) of a region of the earth can be used during geophysical exploration in order to assist with identifying the subterranean formations which may be encountered. Because of the correlation between different types of formations and their electrical properties, an appropriate sampling of data obtained from a region can be used to identify faults, mineral deposits, and the geoelectric structure of the rock bedding beneath the surface. One means of determining these electrical properties is by transmitting an electromagnetic signal downward into the earth and then processing the return signal which is received after reflection from one or more layers within the earth, where each layer is associated with a set of electrical properties.
A "Method and Apparatus for Transmitting Electromagnetic Signals into the Earth from a Capacitor" is described in allowed U.S. Pat. application Ser. No. 07/713,955, filed Jun. 11, 1991, which is the parent application of the present continuation-in-part application, and which is hereby incorporated by reference. The parent application teaches the use of a capacitor to transmit a pulse of electromagnetic radiation having a broad frequency band downward into the earth. The inhomogeneous, dispersive nature of the earth causes the pulse to be partially transmitted and reflected at different depths or layers. This results in a return signal being propagated back upward which can be detected by a similar capacitor.
The processing of the return signal permits the electrical characteristics (such as the relative dielectric constant and complex resistivity) of the earth as a function of depth and the frequency components of the transmitted pulse to be determined. These characteristics can then be correlated to specific types of subsurface formations in order to image the subsurface region. Information about the subsurface formations may then be used to assist with geophysical exploration.
Of background relevance to the present invention are mentioned methods for modeling the propagation of radio waves in the ionosphere to determine the values of the complex indices of refraction for multiple, stratified layers in the propagation medium. For example, the article by J. Ralph Johler (the inventor of the invention described in the present application) entitled "Radio wave reflections at a continuously stratified plasma with collisions proportional to energy and arbitrary magnetic induction", contained in the Proceedings of the International Conference on The Ionosphere held at Imperial College London July 1962, pp. 436-445, published by The Institute of Physics and the Physical Society, and the articles by J. Ralph Johler and John D. Harper, Jr. entitled "Reflection and Transmission of Radio Waves at a Continuously Stratified Plasma With Arbitrary Magnetic Induction", Journal of Research of the National Bureau of Standards, Vol. 66, No. 1, January-February 1962, pp. 81-99, and "On the Effect of Solar Disturbances on the Low-Frequency Ionosphere Reflection Process", contained in The Effect of Disturbances of Solar Origin on Communications, pp. 303-334, G. J. Gassmann editor, published by Pergamon Press, 1963, which are hereby incorporated by reference, discuss a model of the ionosphere which consists of a series of stacked layers of plasma, with each layer being characterized by a complex index of refraction. The propagation of a plane wave through this medium can be modeled according to the methods described in these references and the predicted electron density and collision frequency profiles can be compared to actual measurements as a means of validating the model.
However, the prior art does not teach or suggest how to model the propagation of an electromagnetic signal through the earth to determine electrical properties of interest to geophysical exploration, and does not teach or suggest how to determine such properties in an efficient, iterative manner.